well i agree that your interpretation is easy to get out of St. Athanasius, but based on the quotes you have given i dont think he is necessarily teaching that animals have always been mortal. just as man is mortal by nature but yet was not infected with death from the beginning, the same can be true of animals. true, he connects the possibility for incorruption in man to the image of God, which the animals do not have, but i don't think that what he says excludes the interpretation that the rest of creation is incorrupt by virtue of its connection to the fate of man, as so many other Fathers teach, as creation is seen as man's kingdom and thus connected to his fate. i can only assume that St. Athanasius is familiar with St. Paul's teaching that the creation was subject to futility through man, and creation will be restored through man, so i would think he would have to seen some kind of change in creation due to man's sin, but he doesnt address that in the quotes you provided, so its hard to say what he saw as the change. and i know he at some point references the Wisdom of Solomon 1-2 which tells us that God does not desire the death of anything living, not just man. without further evidence, i can only assume that he teaches in harmony with Saints such as John Chrysostom, Basil the Great, etc.